The use of carrier current signals on alternating current on power distribution systems has been known and utilized for decades in many applications from controlling power distribution circuits to such simple functions as a home remote switch controller.
One of the principle problems concerning carrier current systems is the increasing amount of noise on the AC power lines not only from devices having arcing commutators, i.e., universal motors, centrifugal switch starting motors but also from high frequency noise caused by semiconductor devices such as SCR's and triacs that use phase angle switching. The steep wavefront of these devices creates noise throughout the frequency spectrum. Another source of noise is related to the de-energization of relays, solenoids, vacuum cleaners and food blenders since such devices have severe noise generating characteristics which can occur at any time during the AC power cycle. The application of a square wave, i.e., digital pulse to any conductor including power lines, also causes the generation of radio frequency interference (RFI) across the radio frequency spectrum. This RFI is a serious problem in our present electronic environment as evidenced in the proliferation of devices presently used causing sufficient interference to cause the FCC to impose strict standards on their use, and legislative action has been taken to minimize the problem.
Previous disclosures attempt to minimize the deleterious effects of this noise by transmitting signals at a particular portion of the AC power sinewave.
Another technique is to send repetitive transmissions of the same signal with the assumption that this procedure will provide reliable operation of the system.
Systems previously disclosed implementing binary pulses of short duration (50 microseconds--2 milliseconds) consisting of pulses or frequency shift keyed (FSK) transmissions in a simplex mode cannot cope properly with noise over any appreciable distance and cannot insure the signals will be received or properly interpreted.
Present carrier current devices that utilize digital pulse techniques attempt to avoid causing excessive RFI by limiting the energy impressed on the conductors at the expense of both limiting the range of operation and reducing noise immunity capabilities.
The teachings of the present invention require the observance of certain principles of electronic phenomena that has been well defined and of particular techniques involving the philosophy of communication theory.
The use of electrical conductors transitting 50-60 Hz frequency power on relatively low impedance loads requires for each branch circuit a calculable amount of energy (1-2 Kw) for standard residential wiring, and more for commercial or industrial circuits. This load is usually resistive and/or inductive in nature.
In any given installation, the load on each branch circuit is not constant or equal. Therefore, the impedance of each circuit will change. When applying a carrier frequency on these branch circuits for the purpose of communication or control at, say, two orders of magnitude higher than the power frequency (50-60 Hz) up to the 600-6000 Hz range, the changing load impedance still has an appreciable deteriorating effect. To circumvent any difficulty in this area, a frequency is chosen for the carrier to be high enough to utilize the "Skin Effect" phenomenon. This approach minimizes the power requirements of the carrier oscillators, and insures that the carrier frequency will be able to reach all of the units of the total system. Experimentally, this frequency range is between 150 kHz and 350 kHz. Also important is the characteristic of the carrier frequency waveform. The optimum configuration should be sinusoidal thereby minimizing the creation of a steep wavefront which generates objectional RFI.
The lack of reliability of previously disclosed systems preclude their use in many applications, especially in commercial and industrial systems. Their use in residential applications have not been shown to be entirely successful in extended systems, particularly in a high load, high noise environment.
Other disclosures have recognized and attempted to resolve another problem of AC power carrier current systems, notably the ability to properly supervise the various elements of the system to insure the reliable functioning and operation of the units. This includes the capability to detect any intentional or inadvertent tampering or interference of the units using techniques that will not contribute to false alarms.
As mentioned in previous disclosures, off-normal indications are accomplished by amplitude modulating a carrier used by a plurality of alarm devices. If several alarms occur and are transmitted simultaneously, modulated sum and difference frequencies can occur causing false conditions to exist decreasing the system reliability.
Any attempt to test the integrity of any system units by occasional manual testing does not provide assurance that the system is operable except at the time or occurrence of the test and does not provide continuous supervision.
Any system utilizing amplitude modulated signals is limited in its capabilities, firstly, by the amount of AC line noise which tends to modify the amplitude modulation of the RF carrier thereby limiting the reliability of the system and secondly, by decreasing the range of the RF signal due to severe deterioration caused by low impedance, high load AC circuits.
From the foregoing comments and the following information, an analysis can be made to eliminate many of the deficiencies described above, and in addition, several technical and design improvements will be implemented resulting in a much improved system.
The transmission of information has several important considerations, two of these being speed and accuracy. The speed of data transmission is usually determined by the particular application. Monitoring and control systems in the applications under consideration do not require high speed data transmission, since the response time (transfer function) of the systems are in the range of seconds, not milliseconds, for the functional operation of these electronic and/or electromechanical servo-mechanisms.
The slower the rate of data transmission or signals, the more reliable the data transfer will be since each unit of the data transmission exists for a longer period of time and can be integrated over this time period to verify its existence regardless of the presence of extraneous, co-existing signals composing part of an AC power line carrier environment. The techniques used to provide parity bits, checksums, averaging, or additional data bits to be used for error detection has a minimal effect in a hostile transmission medium except to avoid errors in receiver response but fail to provide reliable operation.
A more realistic approach in this environment would be to receive an identical return signal verifying the transmitted signal prior to terminating the transmitted signal, with the signal consisting of a single unit of information. Since the transmitted signal continues until a return verification signal is received, the receiver has ample time to integrate the received signal and ignore the noise co-existing on the conductors prior to transmitting an identical return signal. This technique would require a minimum of two carrier channels to permit full duplex communication. Using this mode of operation permits the use of a variable transmission time, i.e., a short period for a noise-free AC line, and a long transmission period for a noisy AC line, still maintaining a high degree of reliability.
Also, the RF oscillators required for transmitting signals comprise synchronously gated, stable, sinusoidal oscillators. The output of the oscillator has to be sinusoidal when impressed on the AC line since the assault of a square wave or steep wavefront on any conductor, including power lines, causes the generation of RFI across the radio frequency spectrum. The sinusoidal waveform minimizes the RFI thereby resolving a problem mentioned earlier.
Another serious deficiency of previously described remote alarm and control systems using carrier current signals is concerned with the installation techniques of the various local and remote modules required for the system. The claimed principle advantage of carrier current equipment used in apartment and building alarm or monitoring systems is predicated on the condition that an electrician or electrical contractor will not be required to accomplish the installation. In most urban communities, the local electrical code requires an electrician to perform any electrical modifications on the AC power wiring. Since the previous disclosures (with two exceptions limited to operating appliances) have to be wired into the AC power circuits, this situation drastically upgrades the cost and expertise required to install the alarm or monitoring system presently provided by alarm company installers handling wired low voltage systems either in residential or commercial applications.
The present disclosure permits all of the required low voltage (6-18 v) sensor wiring to be performed and connected to the local and remote modules prior to plugging the modules into AC power duplex receptacles.
Another feature of the present disclosure eliminates a major deficiency in disclosed previous carrier current systems. This deficiency is the omission of any continuous system module supervision to detect any malfunction or tampering to defeat the system automatically without manual intervention or periodic testing.
In any monitoring system, it is essential that the system design is reliable and is not subject to false alarms. The failure, at times, of control systems is not as critical as alarm systems; therefore, the design cannot be marginal and overlook the severe carrier current environments.
The presently disclosed system also provides an automatic correlation program to permit additional modules to be installed in addition to the previously-installed correlated master module group, without requiring any system or fixed program alterations.